S. Noji

Candidate Resonant Tetraneutron State Populated by the He 4 (He 8, Be 8) Reaction

A candidate resonant tetraneutron state is found in the missing-mass spectrum obtained in the double-charge-exchange reaction ^{4}He(^{8}He,^{8}Be) at 186  MeV/u. The energy of the state is 0.83±0.65(stat)±1.25(syst)  MeV above the threshold of four-neutron decay with a significance level of 4.9σ. Utilizing the large positive Q value of the (^{8}He,^{8}Be) reaction, an almost recoilless condition of the four-neutron system was achieved so as to obtain a weakly interacting four-neutron system efficiently.

Spin-isospin selectivity in three-nucleon forces

Abstract Precision data are presented for the break-up reaction, H 2 ( p → , p p ) n , within the framework of nuclear-force studies. The experiment was carried out at KVI using a polarized-proton beam of 190 MeV impinging on a liquid-deuterium target and by exploiting the detector, BINA. Some of the vector-analyzing powers are presented and compared with state-of-the-art Faddeev calculations including three-nucleon forces effect. Significant discrepancies between the data and theoretical predictions were observed for kinematical configurations which correspond to the H 2 ( p → , He 2 ) n channel. These results are compared to the H 2 ( p → , d ) p reaction to test the isospin sensitivity of the present three-nucleon force models. The current modeling of two and three-nucleon forces is not sufficient to describe consistently polarization data for both isospin states.

Neutron single-particle strength in silicon isotopes: Constraining the driving forces of shell evolution

Shell evolution is studied in the neutron-rich silicon isotopes Si36,38,40 using neutron single-particle strengths deduced from one-neutron knockout reactions. Configurations involving neutron excitations across the N=20 and N=28 shell gaps are quantified experimentally in these rare isotopes. Comparisons with shell model calculations show that the tensor force, understood to drive the collective behavior in Si42 with N=28, is already important in determining the structure of Si40 with N=26. New data relating to cross-shell excitations provide the first quantitative support for repulsive contributions to the cross-shell T=1 interaction arising from three-nucleon forces.

New Experimental Studies of Nuclear Spin-Isospin Responses

Possible directions of nuclear spin-isospin response studies are discussed. They includes i) light-ion induced charge exchange reaction studies in inverse-kinematics experiments with RI-beams, ii) studies with heavy-ion stable-beam induced charge exchange reactions, and iii) studies with RI-beam induced charge exchange reactions. Among them, RI-beam induced charge exchange reactions have unique properties which are missing in stable-beam induced reactions and can be used to reveal new aspects of spin-isospin responses in nuclei. Several experiments have been done and are planned with the SHARAQ spectrometer at RI Beam Factory, by utilizing the unique properties.

Low-lying level structure of Cu 56 and its implications for the rp process

The low-lying energy levels of proton-rich Cu56 have been extracted using in-beam γ-ray spectroscopy with the state-of-the-art γ-ray tracking array GRETINA in conjunction with the S800 spectrograph at the National Superconducting Cyclotron Laboratory at Michigan State University. Excited states in Cu56 serve as resonances in the Ni55(p,γ)Cu56 reaction, which is a part of the rp process in type-I x-ray bursts. To resolve existing ambiguities in the reaction Q value, a more localized isobaric multiplet mass equation (IMME) fit is used, resulting in Q=639±82 keV. We derive the first experimentally constrained thermonuclear reaction rate for Ni55(p,γ)Cu56. We find that, with this new rate, the rp process may bypass the Ni56 waiting point via the Ni55(p,γ) reaction for typical x-ray burst conditions with a branching of up to ∼40%. We also identify additional nuclear physics uncertainties that need to be addressed before drawing final conclusions about the rp-process reaction flow in the Ni56 region.

Inverse-kinematics proton scattering on 50 Ca: determining effective charges using complementary probes

A comparison of the present results with those from previous measurements of the lifetimes of the 2 + states provides us the ratio of the neutron and proton matrix elements for the 0 +.s. ! 2 + transitions. These results allow the determination of the ratio of the proton and neutron effective charges to be used in shell model calculations of neutron-rich isotopes in the vicinity of 48 Ca. Isotopes within a few nucleons of the doubly-magic nuclei provide the foundation for the nuclear shell model. The simplicity of the wavefunctions of the valence nucleons in these nuclei allows the determination of the two-body matrix elements necessary for predicting the energies of excited states and the effective charges used in calculating transition strengths in a given shell-model space. Effective charges reflect the strength of the coupling between the motion of the valence nucleons and the virtual excitations of the core nucleons.

Observation of the Isovector Giant Monopole Resonance via the Si 28 (Be 10, B∗ 10 [1.74 MeV]) Reaction at 100 AMeV

The (^{10}Be,^{10}B^{*}[1.74  MeV]) charge-exchange reaction at 100  AMeV is presented as a new probe for isolating the isovector (ΔT=1) nonspin-transfer (ΔS=0) response of nuclei, with ^{28}Si being the first nucleus studied. By using a secondary ^{10}Be beam produced by fast fragmentation of ^{18}O nuclei at the NSCL Coupled Cyclotron Facility, applying the dispersion-matching technique with the S800 magnetic spectrometer to determine the excitation energy in ^{28}Al, and performing high-resolution γ-ray tracking with the Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) to identify the 1022-keV γ ray associated with the decay from the 1.74-MeV T=1 isobaric analog state in ^{10}B, a ΔS=0 excitation-energy spectrum in ^{28}Al was extracted. Monopole and dipole contributions were determined through a multipole-decomposition analysis, and the isovector giant dipole resonance and isovector giant monopole resonance (IVGMR) were identified. The results show that this probe is a powerful tool for studying the elusive IVGMR, which is of interest for performing stringent tests of modern density functional theories at high excitation energies and for constraining the bulk properties of nuclei and nuclear matter. The extracted distributions were compared with theoretical calculations based on the normal-modes formalism and the proton-neutron relativistic time-blocking approximation. Calculated cross sections based on these strengths underestimate the data by about a factor of 2, which likely indicates deficiencies in the reaction calculations based on the distorted wave Born approximation.

SHARAQ Spectrometer—Current status and future experimental plans—

The high‐resolution SHARAQ spectrometer and the dedicated high‐resolution beamline have been constructed at RI beam factory and commissioned in 2009. Lateral and angular dispersion matching conditions were achieved simultaneously for a primay beam with a momentum spread of 0.1% and an emittance of 3π mm⋅mrad (σ). An overview of the spectrometer and the beam‐line, preliminary results from the commissioning runs, and future experimental plan with the system are described.

Measurement of Ay for the pd Breakup Reaction at 250 MeV

From the recent studies, it has been found that the differential cross sections of the elastic Nd scattering at the energy below 150 MeV can be well reproduced by incorporating the 3NF in the Faddeev calculation based on modern nucleon‐nucleon (NN) interactions. On the other hand, the differential cross sections of nd and pd at 250 MeV show large discrepancies between the data and the Faddeev calculations with 3NF. It indicates the presence of the missing features of the three nucleon system at this energy region. For the next step of the 3NF study, we measured the vector analyzing power Ay and the differential cross sections for the 2H(p, pp)n breakup reaction at 250 MeV. The experiment was carried out by detecting two protons in the final state with using the double‐armed spectrometer Grand Raiden (GR) and Large Acceptance Spectrometer (LAS) at Research Center for Nuclear Physics (RCNP). These data provide important information on the 3NF effects and make a discrimination between 3NF models.

Gamma decay of pygmy states in 90,94Zr from inelastic scattering of light ions

We performed experiments to study the low-energy part of the E1 response (Pygmy Dipole Resonance) in